The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against 13C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced 13C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.
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Ancient boreal forests under the environmental instability of the glacial to postglacial transition in the Great Lakes region (14 000 – 11 000 years BP)
Retreat of the Laurentide Ice Sheet 20 000 years ago tremendously altered environmental conditions and opened territory to the boreal spruce forest expansion. However, the details of forest colonization during the rapid climate warming and the adaptation of the newly developed stands to short cooling episodes during the warming and degradation of the ice sheet are not known. Preservation of wood from the glacial to postglacial transition offers the opportunity for examination of high-frequency growth variability in response to hemispheric and local forcings on temperature and hydrology. Here we consider growth of spruce at three sites from the interior of Northern America developed at ca. 13 700, 12 100, and 11 300 calibrated years before present (cal years BP), with well-replicated tree-ring chronologies spanning from 116 to 310 years. The data show at least two generations of trees established at each of the sites promoted by short, warm intervals. The tree mortality was variously affected by both cold conditions and the influence of rising water table and sediment burial. The history of these stands indicates breaks in forest colonization following a century (or two) of successful migrations. Interestingly, the thinning of the spruce forest did not seem to open pioneering opportunities for other tree species at those times.
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- Award ID(s):
- 0213696
- NSF-PAR ID:
- 10343021
- Date Published:
- Journal Name:
- Canadian Journal of Forest Research
- Volume:
- 43
- Issue:
- 11
- ISSN:
- 0045-5067
- Page Range / eLocation ID:
- 1032 to 1039
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Disturbances can interrupt feedbacks that maintain stable plant community structure and create windows of opportunity for vegetation to shift to alternative states. Boreal forests are dominated by tree species that overlap considerably in environmental niche, but there are few tests of what conditions initiate and sustain different forest states. Here, we examine patterns of post‐fire growth and density of tree seedlings in early succession and use structural equation models to estimate relative effects of environmental and pre‐fire conditions, fire characteristics, and biotic interactions. We surveyed tree seedling recruits for 13 yr across a broad range of environmental and fire conditions (
n = 89) in Alaskan black spruce stands that burned in 2004. Densities of established seedlings at 13 yr were strongly determined by initial recruitment that occurred within 2 yr after fire. High proportional combustion of the soil organic layer (fire severity) led to increased densities of deciduous seedlings but not of black spruce and had a positive influence on aboveground biomass of all species. Biotic interactions such as mammalian herbivory or woody competition, potential mechanisms for relay floristic succession, had no detectable effects on tree seedling densities or biomass. Repeated surveys instead suggested persistent shifts in successional trajectories of tree communities from spruce to deciduous dominance at sites where high fire severity created positive conditions for deciduous seedling recruitment and growth. Unless future species interactions alter the deciduous dominance of tree seedling composition, the vegetation transformations that we observed in response to high fire severity are likely to persist over the short fire cycle that increasingly characterizes the fire regime of Interior Alaska. -
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Abstract The spatial overlap of multiple ecological disturbances in close succession has the capacity to alter trajectories of ecosystem recovery. Widespread bark beetle outbreaks and wildfire have affected many forests in western North America in the past two decades in areas of important habitat for native ungulates. Bark beetle outbreaks prior to fire may deplete seed supply of the host species, and differences in fire‐related regeneration strategies among species may shift the species composition and structure of the initial forest trajectory. Subsequent browsing of postfire tree regeneration by large ungulates, such as elk (
Cervus canadensis ), may limit the capacity for regeneration to grow above the browse zone to form the next forest canopy. Five stand‐replacing wildfires burned ~60,000 ha of subalpine forest that had previously been affected by severe (>90% mortality) outbreaks of spruce beetle (SB,Dendroctonus rufipennis ) in Engelmann spruce (Picea engelmannii ) in 2012–2013 in southwestern Colorado. Here we examine the drivers of variability in abundance of newly established conifer tree seedlings [spruce and subalpine fir (Abies lasiocarpa )] and resprouts of quaking aspen (Populus tremuloides ) following the short‐interval sequence of SB outbreaks and wildfire (2–8 yr between SB outbreak and fire) at sites where we previously reconstructed severities of SB and fire. We then examine the implications of ungulate browsing for forest recovery. We found that abundances of postfire spruce seedling establishment decreased substantially in areas of severe SB outbreak. Prolific aspen resprouting in stands with live aspen prior to fire will favor an initial postfire forest trajectory dominated by aspen. However, preferential browsing of postfire aspen resprouts by ungulates will likely slow the rate of canopy recovery but browsing is unlikely to alter the species composition of the future forest canopy. Collectively, our results show that SB outbreak prior to fire increases the vulnerability of spruce–fir forests to shifts in forest type (conifer to aspen) and physiognomic community type (conifer forest to non‐forest). By identifying where compounded disturbance interactions are likely to limit recovery of forests or tree species, our findings are useful for developing adaptive management strategies in the context of warming climate and shifting disturbance regimes. -
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Abstract Much is still unknown about the growth and physiological responses of trees to global change at the northern treeline. We combined tree‐ring width data with century‐long stable carbon and oxygen isotope records to investigate growth and physiological responses of white spruce at two treeline sites in the Canadian Arctic to concurrent increases in temperature, atmospheric CO2concentration (
c a ), and decline in sea ice extent over the past century. The tree‐ring records were assessed during three periods with contrasting climatic conditions: (a) the early 20th century warming, (b) the 1940–1970 cooling period, and (c) the anthropogenic late 20th century warming period. We found opposing growth trends between the two sites, but similar carbon isotope discrimination (Δ13C) and intrinsic water‐use efficiency (iWUE) trajectories. While tree growth (defined as basal area increment) increased at the site nearer to the Arctic Ocean during the 20th century following the rise in temperature and sea ice loss, growth declined after 1950 at the more interior site. At both sites, Δ13C slightly increased over these periods. However, trees showed a nonlinear response to increasedc a , shifting after 1970 from a passive stomatal response (i.e., no changes iniWUE) to an active response (i.e., a moderate ∼12% increase iniWUE). Further, our isotope‐based findings do not support the idea that temperature‐induced drought stress caused the divergent growth trends at our treeline sites. This study thus highlights nonlinear and complex physiological and growth adjustments to concomitant changes in temperature, sea ice extent, andc a over the last century at the northern treeline. -
1. Amplified by warming temperatures and drought, recent outbreaks of native bark beetles (Curculionidae: Scolytinae) have caused extensive tree mortality throughout Europe and North America. Despite their ubiquitous nature and important effects on ecosystems, forest recovery following such disturbances is poorly understood, particularly across regions with varying abiotic conditions and outbreak effects. 2. To better understand post-outbreak recovery across a topographically complex region, we synthesized data from 16 field studies spanning subalpine forests in the Southern Rocky Mountains, USA. From 1997 to 2019, these forests were heavily affected by outbreaks of three native bark beetle species (Dendroctonus ponderosae, Dendroctonus rufipennis and Dryocoetes confusus). We compared pre- and post-outbreak forest conditions and developed region-wide predictive maps of post-outbreak (1) live basal areas, (2) juvenile densities and (3) height growth rates for the most abundant tree species – aspen (Populus tremuloides), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta) and subalpine fir (Abies lasiocarpa). 3. Beetle-caused tree mortality reduced the average diameter of live trees by 28.4% (5.6 cm), and species dominance was altered on 27.8% of field plots with shifts away from pine and spruce. However, most plots (82.1%) were likely to recover towards pre-outbreak tree densities without additional regeneration. Region-wide maps indicated that fir and aspen, non-host species for bark beetle species with the most severe effects (i.e. Dendroctonus spp.), will benefit from outbreaks through increased compositional dominance. After accounting for individual size, height growth for all conifer species was more rapid in sites with low winter precipitation, high winter temperatures and severe outbreaks. 4. Synthesis. In subalpine forests of the US Rocky Mountains, recent bark beetle outbreaks have reduced tree size and altered species composition. While eventual recovery of the pre-outbreak forest structure is likely in most places, changes in species composition may persist for decades. Still, forest communities following bark beetle outbreaks are widely variable due to differences in pre-outbreak conditions, outbreak severity and abiotic gradients. This regional variability has critical implications for ecosystem services and susceptibility to future disturbances.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1139/cjfr-2012-0339
